This invention generally relates to stress-engineered metal films, and more particularly to photo lithographically patterned micro-spring structures formed from stress-engineered metal films.
Photo lithographically patterned spring structures (sometimes referred to as xe2x80x9cmicrospringsxe2x80x9d) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical spring structure includes a spring metal finger having an anchor portion secured to a substrate, and a free (cantilevered) portion extending from the anchored portion over the substrate. The spring metal finger is formed from a stress-engineered metal film (i.e., a metal film fabricated such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The free portion of the spring metal finger bends away from the substrate when the release material located under the free portion is etched away. The internal stress gradient is produced in the spring metal by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. Such spring metal structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. For example, when utilized in a probe card application, the tip of the free portion is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring metal structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
The present inventors have observed that conventional spring structures develop contact resistances that are detrimental to signal transmissions when the spring structures are used as conductors. The spring metal (e.g., Mo, MoCr, NiZr) is typically chosen for its ability to retain large amounts of internal stress. These materials typically oxidize in air, a phenomenon that can interfere with their ability to make electrical contact, for example, with the contact pad of an integrated circuit when used in a probe card. The spring metal materials can also gall to the contact pad, which is typically aluminum. Once the galled aluminum oxidizes, the contact resistance between the contact pad and the spring metal structure increases. One proposed approach to reducing contact resistance is to passivate the spring metal before etching and release. However, the passivating material tends to resist bending of the spring metal finger after release, and provides minimal coverage along the front edge at the tip, thereby allowing direct contact with the spring metal that can result in increased contact resistance.
What is needed is a spring metal structure that resists increased contact resistance by avoiding oxidation of the spring metal and/or galling of a contact pad against which the spring metal structure is pressed.
The present invention is directed to spring structures having passive-conductive coatings formed on a tip thereof, and to methods for fabricating these spring structures.
In accordance with a first embodiment of the present invention, the passive-conductive coating is formed using stress-engineering methods similar to those used to form the underlying spring finger. Like the underlying spring finger, the passive-conductive coating is formed with stress variations in the growth direction such that the passive-conductive coating help the stress-induced bending of the finger during the release process. After release, the passive-conductive coating provides a spring structure with reduced contact resistance when compared to non-coated spring structures.
In accordance with a second embodiment of the present invention, methods for fabricating microspring structures are disclosed in which a conductive coating (e.g., a refractory noble metal such as Rhodium (Rh), Iridium (Ir), Rhenium (Re), Platinum (Pt), and Palladium (Pd)) is deposited on the tip of the free (i.e., cantilevered) portion of the spring metal finger using an intermediate mask that is patterned between the formation (etching) of the spring finger and the release of the spring finger. A first mask is formed over sequentially formed release and spring metal layers that is used to etch the underlying spring metal and release layers to form a spring metal island formed on a release material island. The second mask is then formed with a window that exposes a tip of the spring metal island. In one embodiment, the second mask is photoresist formed with undercut (i.e., negative sloped) walls to facilitate liftoff of the passive-conductive coating formed on the upper surface of the second mask. To prevent the formation of a flange that may undesirably secure (anchor) the tip to the underlying substrate, the spring structure is briefly immersed in a release material etchant to remove the release material located under the tip prior to the deposition of the passive-conductive coating. The passive-conductive coating is then deposited through the second window onto the tip of the spring metal island. In one embodiment, a directional deposition process is utilized to facilitate shadowing. The second mask is then stripped, and a release mask is patterned that defines a window exposing a free end (including the tip) of the spring metal island for release. The structure is again immersed in the release material etchant, causing removal of the release material exposed by the release mask and bending of the exposed free portion of the spring metal island away from the substrate due to its internal stress, thereby becoming the free portion of a spring metal finger (an anchored portion of the spring metal finger remains covered by the release mask). The release mask may then be stripped.